The present invention is in the field of Fischer-Tropsch synthesis, in removing trace sulfur from syngas feeds to Fischer-Tropsch reactors.
Fischer-Tropsch synthesis can be used to convert natural gas to fuels. The process involves the initial conversion of a light hydrocarbon feed to syngas, and then conversion of the syngas to hydrocarbon products in a Fischer-Tropsch reactor. Fischer-Tropsch catalysts used to convert synthesis gas to hydrocarbons are poisoned by traces of sulfur. This is of particular concern in Fischer-Tropsch reactors that feature fluidized beds.
Numerous references have acknowledged the need to keep sulfur levels low in synthesis gas. For example, U.S. Pat. No. 5,968,465 teaches that the feed gas must be essentially free of sulfur, and defines xe2x80x9cessentially freexe2x80x9d as 10 wppm or less, preferably 5 wppm or less, more preferably, 1 wppm or less, and most preferably about 50 wppb. U.S. Pat. No. 5,882,614 teaches that the total sulfur should be less than 10 vppb. In rare cases, sulfur can be added to Fischer-Tropsch catalysts, but this causes a shift in product distribution to light products which in general are not as desirable as heavy products such as distillate fuels and lube base stocks.
Techniques for removing sulfur from syngas before Fischer-Tropsch synthesis are well known. They typically involve using a caustic solution (typically an amine) and adsorption on a support (typically a metal oxide, for example, zinc oxide). Examples of these techniques are described, for example, in U.S. Pat. Nos. 4,088,735 and 3,941,820.
The metals in Fischer-Tropsch catalysts strongly adsorb sulfur. Virtually every atom of sulfur that enters the reactor will attach to a catalytically active site and poison it. Lowering the concentration of sulfur in the syngas will significantly enhance the lifetime of the Fischer-Tropsch catalyst.
The following calculation shows the importance of removing sulfur from the syngas feed. Assuming the Fischer-Tropsch catalyst contains 20 wt % cobalt (3.39 micromoles/gram) and that 10% of the cobalt atoms exposed to the syngas are catalytically active and 90% of the atoms are not accessible to the syngas and are inactive, the amount of surface cobalt atoms is 0.339 micromoles/gram. With a syngas molar ratio of 2H2 per CO, a total CO rate of 1000 cc/g(cat)/h (3000 cc/g(cat)/h) and a syngas per-pass conversion is 60%, the GHSV of fresh syngas is 5000 cc/k(cat)/h. This is equivalent to 120,000 cc/g(cat)/day and 43,800,000 cc/g(cat)/year.
If a catalyst is considered inactive if 90% of the surface sites are destroyed, with 0.339 micromoles/g of surface sites, 0.305 micromoles must be destroyed for the catalyst to be inactive. Based on lab studies, each sulfur atom deactivates approximately six surface cobalt atoms, so only 0.0509 micromoles/g of sulfur is needed to deactivate this typical catalyst.
If the sulfur in the syngas is present as hydrogen sulfide at 1 part per billion by weight, with a syngas density of 10.67 g/mole and an H2S density of 34 g/mol, 1 part per billion of sulfur (as H2S) is equivalent to 0.31 parts per billion by volume. With the GHSV of the fresh syngas above, the equivalent H2S GHSV are shown below.
Accordingly, if the catalyst can adsorb 0.0509 micromoles of sulfur before becoming inactive, it will last approximately 83 years. The following table correlates the catalyst life and the feed sulfur levels using the above assumptions.
In general, it is desirable to have the Fischer-Tropsch catalyst last as long as possible. 1 year life is unacceptable, and lives approximately of 5 years or greater are needed for acceptable use. This requires that the feed sulfur be consistently below an approximate maximum of 20 ppbw.
U.S. Pat. No. 5,968,465 specifies that the feed sulfur should be below 50 wppb, which may not be acceptable for some catalysts. U.S. Pat. No. 5,882,614 teaches that the syngas to the FT unit should be below 10 ppmv, but achieves this level by treating the feed to the upstream syngas generation unit. This method is not totally reliable, and other methods to reaching low levels of sulfur in syngas are needed.
It is impossible to completely eliminate all sulfur from the feed, and some contamination is inevitable. However, since the Fischer-Tropsch catalysts and processes are valuable, it would be desirable to have methods for keeping sulfur levels as low as possible. It would be advantageous to provide methods for improving the ability of a syngas conversion facility to tolerate sulfur in the Fischer-Tropsch section. The present invention provides such methods.
Methods for removing sulfur from syngas in a Fischer-Tropsch reactor, and reactors including means for removing sulfur from syngas are disclosed.
The present invention provides an apparatus for a Fischer-Tropsch process comprising: a gas inlet for conducting an inlet gas stream, and at least one product outlet with a reactor there between including a Fischer-Tropsch catalyst, the reactor operable at temperatures of from 175xc2x0 to 325xc2x0 C., and a pressure from 1 to 20 atmospheres, and a material, included within the inlet gas stream and upstream from the catalyst, capable of binding sulfur contained in the inlet gas stream. Trace amounts of sulfur can be removed by this method.
In one embodiment, sulfur-reactive metals are used in the Fischer-Tropsch unit to sequester the sulfur. This can be accomplished in several ways. For example, the Fischer-Tropsch unit can be run in stages. The first stage will adsorb all the sulfur and only the catalyst in this unit will need to be changed. The catalyst in this unit can be made in a less expensive form than the catalysts in later stages. For example, iron can be used as the catalyst in the first stage, and more expensive cobalt and/or ruthenium used in later stages. The catalysts in the latter stages will still age slowly by other mechanisms (e.g. metal sintering, forming alloys between the metal and the support). Portions of the aging catalysts in the latter beds can be routed to the first bed to act as a sulfur trap. Preferably, the Fischer-Tropsch catalyst in the first reactor is less active than the catalysts in the later beds. Since the Fischer-Tropsch reaction is highly exothermic, it is most difficult to control in the early stages of the reaction when a large amount of reactive syngas is still present. Once part of the syngas has been consumed, and the partial pressure of reactants has been reduced, the reaction is easier to control. Accordingly, having a less active partially sulfur-poisoned catalyst in the first bed improves the ability to control the overall process.
In another embodiment, the Fischer-Tropsch reactor includes internal baffles that separate the reactor into zones. For example, the zones can be arrayed in concentric circles, with catalysts sequestered into each zone, and reactants routed from one zone to the other.
In a third embodiment, sulfur adsorbents are placed in the inlet gas manifold. These adsorbents can be inexpensive metals that have a great affinity for adsorption of sulfur (molybdenum, zinc, lead, etc.) but which do not alter the syngas substantially (little activity for coke formation, water gas shift reaction, methanol synthesis, or Fischer-Tropsch synthesis.)
In a fourth embodiment, part of the Fischer-Tropsch catalyst is converted into larger size pellets. These will not fluidize with the finer grain Fischer-Tropsch catalyst and will remain near the gas inlet where they will act to adsorb and sequester the sulfur. These larger size pellets can be separated from the finer grain material using a simple sieving device. The large pellets can be made from a Fischer-Tropsch catalyst that does not quite meet specifications and which would otherwise have to be reprocessed. The larger pellets can also include a non-Fischer-Tropsch active metal that has a high affinity for sulfur.
These embodiments can be combined in any suitable manner to lower the sulfur concentration in the syngas feed. The resulting syngas feed preferably has a sulfur concentration less than 50 wppb, more preferably less than 15 wppb, and most preferably less than 1 wppb.